Creep Analysis of Steel Pipe Concrete Axially Compressed Members

Literature Overview

The paper by Yang Yang, Wang Qicai, Zhang Rongling, Cui Jianlong, and Ge Yong, published in Sichuan Building Science (Volume 41, Issue 2, 2015, pages 70-72), presents a creep analysis of steel pipe concrete axially compressed members. Funded by the Changjiang Scholars and Innovative Research Team Development Program (IRT1139) and the Lanzhou Jiaotong University Young Science and Technology Fund (2012028), this research was conducted at Lanzhou Jiaotong University. The topic addresses a critical long-term behavior issue in steel pipe concrete structures, as creep-induced stress redistribution can significantly affect the long-term load-bearing capacity and serviceability of these composite members.

Core Technical Content and Methodology

The authors developed a stress redistribution calculation method for steel pipe concrete axially compressed members by introducing a concrete equivalent elastic modulus approach. Starting from the deformation compatibility condition between the steel pipe and the concrete core, the method accounts for the differential creep behavior of the two materials under sustained axial loading. The steel pipe does not creep under sustained load (within the elastic range), while the concrete core undergoes time-dependent deformation, leading to progressive stress transfer from the concrete to the steel pipe.

The equivalent elastic modulus approach modifies the elastic modulus of concrete to account for the time-dependent deformation, effectively transforming the creep problem into a modified elastic problem. This approach simplifies the mathematical formulation while maintaining reasonable accuracy for engineering applications. The method explicitly considers the steel ratio, stress level, and material grade as key influencing factors.

Stress Redistribution Mechanism

Under sustained axial loading, the concrete core undergoes creep deformation while the steel pipe remains elastically deformed (assuming stresses remain below the yield point). This differential deformation causes a progressive transfer of stress from the concrete to the steel pipe. The stress redistribution continues until a new equilibrium is reached, which may take years depending on the loading conditions and material properties.

The magnitude of stress redistribution is governed by the ratio of the creep coefficient of concrete to the elastic modulus ratio between steel and concrete. A higher creep coefficient (due to lower concrete grade, higher stress level, or lower humidity) results in greater stress transfer to the steel pipe. Conversely, a higher steel ratio provides more capacity to absorb the transferred stress, potentially delaying the onset of steel pipe yielding.

The analysis reveals that for typical steel pipe concrete members with steel ratios of 10-15%, the stress redistribution can increase the steel pipe stress by 10-20% over a period of 5-10 years. This is a significant effect that must be accounted for in long-term design, particularly for structures subjected to sustained high loads.

Engineering Practice Implications

In practice, creep-induced stress redistribution affects several aspects of steel pipe concrete member design. First, the long-term load-bearing capacity may be reduced if the steel pipe stress approaches the yield point due to stress transfer. Second, the differential deformation between the steel pipe and concrete can lead to additional interfacial stresses that may affect the bond integrity. Third, creep contributes to long-term deflection in columns, which affects the overall structural stability and may lead to additional second-order effects (P-Δ effects).

Design recommendations based on this study include: using higher grade concrete for members subjected to high sustained loads to reduce creep, limiting the initial stress level in the concrete core to below 0.6 times the compressive strength, incorporating the stress redistribution effect into the long-term capacity assessment, and considering time-dependent deformation in the serviceability verification of columns. For critical structures, long-term monitoring of strain and deflection is recommended to validate the analytical predictions.

Study Insights and Implications

This research provides a practical and rational method for analyzing the creep-induced stress redistribution in steel pipe concrete axially compressed members. The key insight is that creep is not merely a deflection issue but also a stress redistribution phenomenon that can significantly affect the long-term load-bearing capacity of the composite member. The equivalent elastic modulus approach offers a computationally efficient method that captures the essential physics of the problem. For engineers designing steel pipe concrete structures, the practical implication is that long-term behavior must be explicitly considered in the design process, particularly for members subjected to sustained high axial loads. The study also highlights the importance of material selection and stress level control as effective strategies to mitigate the adverse effects of creep.